Field of the Invention
This invention relates to a method of driving a display device, and more particularly to a method of driving a display device commonly known as an active matrix display device wherein an active element and a display element are provided for each picture element.
In recent years, display devices of an active matrix type have been developed very popularly. This type of display devices is disclosed, for example, in an article by Bernard J. Lechner et al, pp. 1566 to 1579, PROCEEDINGS OF THE IEEE, Volume 59. This type would enable matrix display of a high density even with display devices having a bad threshold characteristic such as liquid crystal by incorporating an active element such as a transistor or a non-linear resistor element having a chargeable facility in corresponding relationship to a display element of each picture element. Methods of driving a display device of the type include principally an element-at-a-time addressing method and a line-at-a-time addressing method, which are both already known by the above mentioned article by B. J. Lechner et al (p. 1570). The element-at-a-time addressing method is disclosed in detail in Japanese Laid-Open Patent Application No. 49-74438, especially with reference to FIG. 6 and so on of the patent. Both known driving methods are now described with reference to FIGS. 1 to 3 of the accompanying drawings.
Referring first to FIG. 1 which is a block circuit diagram of a conventional display device, reference numeral 1 designates a display section which includes row electrodes C.sub.1, C.sub.2, . . . , C.sub.N and column electrodes R.sub.1, R.sub.2, . . . , R.sub.M, and active elements 2 and display elements 3 disposed corresponding to crossing points of the row and column electrodes. Reference numeral 8 designates a row driving section which supplies time-divided scanning signals to the row electrodes. Reference numeral 9 designates a column driving section which supplies data signals to the column electrodes. In the case of the element-at-a-time addressing method, the row driving section 9 includes a sampling pulse generating circuit 7, and switching gates 6 and storage capacitors 5.
Referring to FIG. 2 which is a timing chart of the device of FIG. 1, a serial video signal VD is transmitted to the display elements of the individual picture elements in response to sampling pulses S.sub.1, S.sub.2, . . . , S.sub.M and scanning signals C.sub.1, C.sub.2, . . . , C.sub.N.
Now, the driving method by line-at-a-time addressing will be described with reference to FIG. 3.
The device of FIG. 3 is different at a portion of the column driving section 9 indicated at 30 from that of FIG. 1. In the element-at-a-time addressing method, a video signal VD is transmitted directly to the column electrodes by the switching gates 6, but in the line-at-a-time addressing method, a video signal VD is latched into first analog latches 31 in response to operation of the sampling pulse generator 7 and then latched collectively into second analog latches 32 for establishing an intended time relationship. The signals thus latched are then supplied to the individual column electrodes through respective amplifiers 33.
Comparison is now given between the element-at-a-time addressing method and the line-at-a-time addressing method.
First, an advantage of the element-at-a-time addressing method is that circuitry therefor is simple in construction. In particular, the element-at-a-time addressing method eliminates the necessity of two stages of analog latches which are necessitated in the line-at-a-time addressing method. Normally, an analog latch of high speed, high resolution and low distortion features can be obtained but with difficulty, and hence, in this point, the element-at-a-time addressing method is more advantageous than the line-at-a-time addressing method. On the other hand, an advantage of the line-at-a-time addressing method is that even active elements of a display section which have rather low responsiveness and a rather bad threshold level characteristic can be utilized therefor. For a display including 500 rows and 500 columns and having a frame synchronization of 60 Hz, in the case of the line-at-a-time addressing method on one hand, a period of time of one 500.times.60th second (33 microseconds) is sufficient for a writing operation while in the case of the element-at-a-time addressing method on the other hand, a writing operation must be done within a period of time of one 500.times.500.times.60th second (66 nanoseconds) in the minimum.
Further, since in the case of the element-at-a-time addressing method electric charges are stored in wiring capacitors and storage capacitors, periods of time actually required for writing operations for individual column are differentiated from one another, and in the example of FIG. 1, they become longer toward the leftmost column and become shorter toward the rightmost column.
In summary, the element-at-a-time addressing method is advantageous in simpler construction of circuitry therefor than the line-at-a-time addressing method while the former has a defect that a period of time required for a writing operation must be short and is not constant or uniform.
In the meantime, typical ones of active elements which can be used in an active matrix are TFTs (thin film transistors) and non-linear resistor elements, and in most cases, a-Si (amorphous silicon) is used as a material for them.
For example, at present a-Si TFTs are studied in more than twenty research organizations in Japan. However, low mobility of carriers thereof makes it difficult to effect a writing operation in a period of time less than 1 microsecond, and hence, they cannot be well applied to the element-at-a-time addressing method. Further, an a-Si diode ring has a large driving capacity and it is not impossible to attain a writing operation in 10 nanoseconds, but since a threshold level characteristic is not completely established, a difference in contrast may be caused to appear due to a difference of writing periods of time, and hence, it cannot be well applied to the element-at-a-time addressing method.